Best way to add a face if I modeled the body first?
I would say that it depends on your level of detail. If your face is mostly a texture and you can get the geometry you need by just moving vertices around on the body, then I would take that route.

But if you need to add significant amounts of geometry, then you will need to change your body geometry to keep good topology. See this article from cg cookie.

Topology (basically having your edges flow cleanly) is important for animation and texturing. Too keep good topology you will probably need to modify your body's geometry. Whether you want to split edges as needed and keep the body mesh, or just start over with a new face mesh, is up to you.